1. Field of Invention
The present invention relates to improved methods for the preparation of organometallic complexes, more particularly, to the preparation of cyclopentadienyliron (II) arene complexes, useful as photoactivatable catalysts for a variety of polymerization reactions.
2. Background
Methods for preparation of cyclopentadienyliron (II) arene complexes rely almost exclusively on the ligand exchange reaction of eta-5-dicyclopentadienyliron (II) (commonly referred to as ferrocene) with a arene in the presence of a Lewis acid. This reaction is described by equation I ##STR1## where Cp is the cyclopentadienyl anion, Ar is a arene and L is a Lewis acid.
This reaction reportedly involves removal of one cyclopentadienyl anion ligand from ferrocene by the Lewis acid as described and whose structures are given as examples in the Journal of Organometallic Chemistry Library 1977, 3, 311 and Tetrahedron 1983, 39, 4037. It is theorized that the referenced reaction produces a coordinately unsaturated cyclopentadienyliron (II) cation and a cyclopentadienyl anion-Lewis acid complex (anion-acid complex).
The cyclopentadienyliron (II) cation then coordinates the arene to give the cyclopentadienyliron (II) arene complex product. The cyclopentadienyl anion-Lewis acid complex undergoes further chemistry, the nature of which depends on the particular Lewis acid used. However, in none of the reactions known in the related art does this complex contribute to the formation of cyclopentadienyliron (II) arene cation.
The most commonly used Lewis acid for these reactions is aluminum chloride (AlCl.sub.3). Additionally, aluminum bromide, gallium chloride, zirconium tetrachloride, hafnium tetrachloride, boron trifluoride and tin tetrachloride may also be used. Mixtures of zirconium or hafnium tetrachloride with aluminum chloride and titanium tetrachloride have also been described (See EP-A 314,618 and U.S. Pat. No. 4,868,288).
Furthermore, it has been reported that the ligand exchange reaction is often benefited by addition of a reducing metal. Using a reducing metal not only increases product yield, it advantageously reduces the amount of side reactions typically associated with ligand exchange. The most commonly used reducing metal is a finely divided aluminum powder. Examples and details of reactions using a reducing metal are illustrated in Tetrahedron 1983, 39, 4037. Reportedly, addition of a small amount of water to aluminum chloride catalyzed reactions can improve product yield. Id. at 4037.
When ligand exchange with aluminum chloride is performed pursuant to the prior art methods, the intermediate cyclopentadienyl anion (also referred to as a cyclopentadienide)-aluminum chloride complex (anion-acid complex) is unstable under the reaction conditions. Further reaction leads predominantly to polymeric material and ferrocene alkylation products. (See Journal of Organometallic Chemistry, 1976, 111, 339.)
However, when the Lewis acid used is zirconium or hafnium tetrachloride, the cyclopentadienide-Lewis acid complex (anion-acid complex) is converted to a stable, isolatable organometallic complex. For example, using zirconium tetrachloride leads to the formation of zirconocene dichloride. (See EP-A 314,618).
In several prior art references, the ferrous ion is used in related reactions. For example, the Tetrahedron article supra discloses, in a related reaction, that the ferrous or ferric ion, in the presence of a Lewis acid catalyst, can coordinate two molecules of a neutral arene to form a bis-arene iron (II) di-cation. This reaction is an example of the generally known Fischer-Haffner reaction. Id. at 4037. The Lewis acid most commonly used in the Fischer-Haffner reaction is aluminum chloride and is shown in equation II. ##STR2##
In another example, it is taught that ferrocenes may be prepared by reaction of cyclopentadienide salts with ferrous ion. This method of preparing ferrocene is described in Organic Syntheses Collective Volume IV, Rabjohn, N. Ed.; Wiley, New York, 1963, p. 473. The preparation of ferrocene as described and shown in equation III, is usually run using sodium cyclopentadienide and ferrous chloride in cold tetrahydrofuran. In this preparation method however, a Lewis acid catalyst is typically not required. EQU Fe.sup.++ +2Cp.sup.- .fwdarw.(Cp).sub.2 Fe (III)
However, none of these prior art references teach transferring a cyclopentadienyl anion ligand from the Lewis acid complexes to a ferrous ion.